Backfeeding transformers with grid interactive micro-inverters

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Nothing that I know of.

Thanks.
You said-

2. The M250 will either provide 240V nominal line to line or 208V nominal line to line. It does not source current through the neutral at all in either configuration.

If the inverters are connecting to a wye transformer, doesn't this sort of connection pictured mean the neutral (bottom line) is connected to the center of the Y (and also grounded?), so the 3 phases are all 120V?
This is three 4 wire 3 phase inverters connected to grid.
Capture.PNG
 

Smart $

Esteemed Member
Location
Ohio
Current sources don't need to raise voltage to push current. There is voltage drop between an inverter and the service, but the inverter doesn't need it to function; it's the effect of current flow, not the cause. An inverter would be perfectly happy feeding superconductors with zero resistance and therefore zero voltage rise.
But in an everyday environment, reverse power flow requires voltage rise to overcome the same transfer impedances encountered with normal [forward?] power. There is no way around it if you want the inverter output to get out on the grid.

Simple electrical theory coupled with some practicality. Let's call voltage at POCO service trannie terminals ES and voltage at PV inverter terminals EP (no transformers in between). If ES ≥ EP, can power be transferred to the grid?
 
...and the difference is... what?

If you connect a 480/277V Tripower to a bank of 3 single phase xfmrs, you are supposed to connect L1, L2 and L3 from the inverter, each to an X1 of each 1 ph xfmr, and the neutral to X2 on each xfmr.
Those would be 277V transformers on the inverter side.
If you connect a Tripower to a single 3 phase xfmr, you are connecting L1 L2 and L3 to the outside points of the Y and the neutral to the center of the Y, so that's also 277V on each leg.

You can't take L1 and L2 from the inverter and connect to a xfmr at 480V- then there's no neutral, and the inverter doesn't like that.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
The M250 will either provide 240V nominal line to line or 208V nominal line to line. It does not source current through the neutral at all in either configuration. The N terminal is just there to allow it to monitor the balance of the grid 120/240 so that it can shut down if there appears to be a grid problem (like lost neutral to the house.)

The M250 and M215 also measure voltage ratio and phase angle to determine whether they're supposed to output in the 240V or 208V window.

I believe that this check is UL required now for inverters with L-L output into a 120/240V three wire single phase service.

I don't believe it is, but if you want the inverter to auto-sense whether it's connected to 240V or 208V then you need the neutral. Some inverters can be programmed to either 240V or 208V, in which case the neutral is not required. Other inverters (e.g. ABB) don't have any provisions for that and require the neutral.

I believe the UL standard just specifies parameters for grid monitoring, with no guidance on method. Maybe Enphase and ABB just don't have algorithims that work well without the neutral. Or maybe with Enphase it's just too clunky to program that stuff through the Envoy.

Thanks.
You said-

2. The M250 will either provide 240V nominal line to line or 208V nominal line to line. It does not source current through the neutral at all in either configuration.

If the inverters are connecting to a wye transformer, doesn't this sort of connection pictured mean the neutral (bottom line) is connected to the center of the Y (and also grounded?), so the 3 phases are all 120V?
This is three 4 wire 3 phase inverters connected to grid.

You can talk about L-L phases and L-N phases separately, if you want. The L-L phases are 208. The L-N phases are 120. Some people might say that only the L-L phases are legitimately called phases, I don't really agree.

Enphase inverters are 2 or 3-wire inverters. An M250 or M215 connects to one L-L phase. The C250 connects to one L-N phase.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
If you connect a 480/277V Tripower to a bank of 3 single phase xfmrs, you are supposed to connect L1, L2 and L3 from the inverter, each to an X1 of each 1 ph xfmr, and the neutral to X2 on each xfmr.
Those would be 277V transformers on the inverter side.
If you connect a Tripower to a single 3 phase xfmr, you are connecting L1 L2 and L3 to the outside points of the Y and the neutral to the center of the Y, so that's also 277V on each leg.

You can't take L1 and L2 from the inverter and connect to a xfmr at 480V- then there's no neutral, and the inverter doesn't like that.
So what? Lots of three phase inverters have to have a neutral to operate, and even the ones that don't will not operate with only two phases connected.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
But in an everyday environment, reverse power flow requires voltage rise to overcome the same transfer impedances encountered with normal [forward?] power. There is no way around it if you want the inverter output to get out on the grid.

Simple electrical theory coupled with some practicality. Let's call voltage at POCO service trannie terminals ES and voltage at PV inverter terminals EP (no transformers in between). If ES ≥ EP, can power be transferred to the grid?
Like I said, we ran around this tree ad nauseam a while back.
 

Smart $

Esteemed Member
Location
Ohio
Like I said, we ran around this tree ad nauseam a while back.
Tree's still standing :p

Voltage rise is a commonly misunderstood concept among PV installers, especially those who have not worked on electrical distribution systems with parallel power supplies, such as a utility-interactive PV system. When a grid-connected inverter produces ac current, the impedance from the grid and inverter output-circuit conductors causes an increase in voltage at the inverter relative to the utility voltage. This phenomenon is commonly referred to as voltage rise.

SOURCE: http://solarprofessional.com/articl...iderations-for-utility-interactive-pv-systems
 
So what? Lots of three phase inverters have to have a neutral to operate, and even the ones that don't will not operate with only two phases connected.

I am hoping to get some insights on using a 150KVA - 480V(Delta) / 208V/120V (Wye) dry type transformer in a PV system

I don't get why the OP would use a delta xfmr here, when the service is 480/277 wye.
Seems like delta services are a thing of the past in many places- that's because delta causes issues with (I think) both voltage fluctuation and harmonic...flux?
No idea if the OP is dealing with linear or non-linear loads, but...

I'm thinking, if my POCO *might* require "3 ph zero flux" topology when a delta xfmr is involved... why bother? (yes, I know, I'm not really qualified to use the word topology, just go with it for a minute)
Say you had a Tripower and 208/120 service - you could connect the 480/277 TP to a 480/277 >>>> 208 delta xfmr...but why would you want to if the service was wye?

Harmonic Mitigating Transformers with zero sequence flux cancellation technology are specifically designed to treat the harmonics generated by computer equipment and other non-linear, power electronic loads.
Harmonic mitigating transformers are superior to K-Rated and conventional transformers in reducing voltage distortion (flat-topping) and power losses due to current harmonics created by single-phase, nonlinear loads such as computer equipment. Secondary windings are designed to cancel zero sequence fluxes and eliminate primary winding circulating currents. They treat zero sequence harmonics (3rd, 9th and 15th) within the secondary windings and 5th and 7th harmonics upstream with an appropriate phase shift.

http://www.temcoindustrialpower.com/products/Transformers_-_General/HT0773.html

The only way I can explain it to myself is that it's like feedback from an electric guitar amp, but we're not Neil Young and it's not a good thing, the xfmr might get certain frequencies going in circles, but there's no speaker, it just buzzes louder and heats up- not good when the goal is electrical efficiency instead of Cinnamon Girl!
 
Here's a "hypothetical" question, or perhaps thought.

The OP (or anyone else) isn't just backfeeding the xmfr, it'll be running in both directions.
So you'd want to get a xfmr based on which way it'll be running the majority of the time.

There are the three options:
A. the PV is limited by budget or space and the load is larger (say 2x larger)
B. the system is sized to equal the load as best as possible
C. the load is small so the PV is larger (say 2x again)

So speaking from the load/PV side of things, you'd want to-
A. get a 480/277 to 208/120 step down xfmr- it'll be running towards you most of the time ("you" meaning PV/load side)
B. also get the same as A, because it'll still be running towards you more than 50% of the time, due to the PV output curve falling below load in ealry AM/late PM.
C. get a 208/120 to 480/27 step UP xfmr, because it'll be running AWAY from you the majority of the time (towards the grid side)

If you are causing some sort of "small problem" that you can sort of live with by running a xfmr in reverse, isn't it still "best practice" to have that small problem occur the least amount of the time?
 
Voltage rise is a commonly misunderstood concept among PV installers, especially those who have not worked on electrical distribution systems with parallel power supplies, such as a utility-interactive PV system. When a grid-connected inverter produces ac current, the impedance from the grid and inverter output-circuit conductors causes an increase in voltage at the inverter relative to the utility voltage. This phenomenon is commonly referred to as voltage rise.

So that's why you have the taps on the grid side of any xfmr which is stepping up the inverter voltage then?
If you have these Enphases which are supposed to put out 208/120, but they are putting out 5% over-voltage due to this phenomenon, you'd want to...
connect grid side to a 198V tap, then the Enphases would end up putting out 207.9V?
 

Smart $

Esteemed Member
Location
Ohio
So that's why you have the taps on the grid side of any xfmr which is stepping up the inverter voltage then?
If you have these Enphases which are supposed to put out 208/120, but they are putting out 5% over-voltage due to this phenomenon, you'd want to...
connect grid side to a 198V tap, then the Enphases would end up putting out 207.9V?
No.

Transformer voltage taps were included on many transformers way before the concept of PV systems were even a concept let alone coupled with the grid. Taps are there to mostly to compensate for voltage drop on the [grid] side.

I'd only change voltage taps if the grid voltage was on the fringe of acceptable tolerance.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
If you have these Enphases which are supposed to put out 208/120, but they are putting out 5% over-voltage due to this phenomenon
If they were putting out 5% over-voltage due this phenomenon, that would be as measured at the inverter output terminals. Measured at the service entrance, the voltage would match the service--the 5% represents the voltage drop along the wiring between the inverter and the service. [Which sounds a bit high, so maybe that wiring is undersized.]

Cheers, Wayne
 

Ok, this is my final guess then- that's what all this is about.
The phenomenon is not a problem because of the adjustable parameters?

Pages 39 and 40:
http://files.sma.de/dl/21561/STP12-24TL-US-IA-en-16W.pdf

8.6 Adjustable Parameters
You can change the following parameters for voltage and frequency monitoring with a
communication product (e.g. Sunny Explorer) (see Section 8.2 "Changing Operating Parameters",
page 38).
Name Value/range Default value
Voltage monitoring median maximum threshold(Overvoltage/Fast)
277.0 V to 332.4 V
Default- 332.4 V
Voltage monitoring lower maximum threshold(Overvoltage/Slow)
277.0 V to 332.4 V
Default- 304.7 V
Voltage monitoring upper minimum threshold(Undervoltage/Slow)
138.5 V to 277.0 V
Default- 243.8 V
Voltage monitoring of median minimum threshold(Undervoltage/Fast)
138.5 V to 277.0 V
Default- 138.5 V
Voltage monitoring median max. threshold trip.time(Overvoltage/Fast, maximum time)
0.1 s to 6,000 s
Default- 0.16 s
Voltage monitoring lower max. threshold trip. time(Overvoltage/Slow, maximum time)
0.1 s to 6,000 s
Default- 1.0 s
Voltage monitoring upper min. threshold trip. time(Undervoltage/Slow, maximum time)
0.1 s to 1,000 s
Default- 2.0 s
Voltage monitoring median min. threshold trip.time(Undervoltage/Fast, maximum time)
0.1 s to 1,000 s
Default- 0.16 s
 

Smart $

Esteemed Member
Location
Ohio
Ok, this is my final guess then- that's what all this is about.
The phenomenon is not a problem because of the adjustable parameters?
Well it could be a problem if the inverters didn't have the adjustable parameters... but the phenomenon is [partially] the reason they include adjustable parameters.
 
Well it could be a problem if the inverters didn't have the adjustable parameters... but the phenomenon is [partially] the reason they include adjustable parameters.

Great, thanks.

Here's another question for you or anyone else, loosely based on the title of the thread.
There are two bits of info provoking the question- my POCO says that 4160V service is "no longer standard but may still be available".
They also say that for => a certain size PV system which involves service upgrade (which happens to be the precise size proposed in one case) the customer "may be charged for *some* of the cost of the xfmers.
oh,wait, 3 bits, they also ask "is the customer supplying a xfmr" on the application.

So say the grid is 12470 L-L and 7200V L-N.
I'm assuming the possible 4160V service is delta, because it doesn't say possibly 4160/2400V.
I'm also assuming that when 4160V delta was available, it was supplied by 7200V wye.

One thing which IS available is a high-efficiency 480/277V (which is the voltage of the PV inverters and a standard POCO service) to 4160 delta xfmr.

If standard 480/277 service is supplied by a 12470V delta to 480/277V xfmr setup, would a 7200V L-N/grid >>> 4160 delta to 480/277 wye xfmr setup have any advantages to it?

That way the customer would be paying for the 4160 to 480/277 xfmr, and the grid would be taking care of the 7200V wye to 4160 delta end, however they did that when it was standard.

I'm not sure what the POCO uses for standard 12470V to 480/277 service xfmrs, but if the customer is going to pay for something, might as well be something...better?
Wouldn't a high-efficiency xfmr in that spot send more of the PV output to the grid overall, and pay for itself after....not too long?
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Wouldn't a high-efficiency xfmr in that spot send more of the PV output to the grid overall, and pay for itself after....not too long?
If the transformer is on the grid side of the meter, its efficiency doesn't affect what the meter reads. So I would think it would be economically better for you to get a POCO service that matches your PV inverter output. That way you don't have any transformers (and their associated losses) on your side of the meter.

Perhaps someone else can chime in, I'm way beyond my experience here, this is just theory absorbed by osmosis.

Cheers, Wayne
 
If the transformer is on the grid side of the meter, its efficiency doesn't affect what the meter reads. So I would think it would be economically better for you to get a POCO service that matches your PV inverter output. That way you don't have any transformers (and their associated losses) on your side of the meter.

Perhaps someone else can chime in, I'm way beyond my experience here, this is just theory absorbed by osmosis.

I don't know, seems like there could be some current lost on the neutral, or maybe other kinds of "anomalies".
If someone goofed and hooked up a 208V inverter to a xfmr with the 203 tap going out- that would affect the PV output.

Frequency problems cause inverter to throttle down...
At the bottom of page 14 here, there's something about harmonics and neutral current loss.

I'm trying to absorb theory also!

http://www.iea-pvps-task10.org/IMG/pdf/rep10_06.pdf
 
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